Optical transmission systems with an optical transmitter, an optical waveguide and optical receiver, the optical transmitter comprising an electrical-to-optical transducer, and the optical receiver comprising an optical-to-electrical transducer and a decision circuit are known, e.g., from EP-A2-554 736 corresponding to U.S. Pat. No. 5,371,625. There, an electrical-to-optical transducer at the transmitting end emits an optical signal, e.g., frequency-modulated by an electric digital signal. The optical signal is transmitted over an optical waveguide to an optical receiver having an optical-to-electrical transducer and a decision circuit. With the decision circuit the digital signal is recovered.
It is desirable to increase the transmission speed of digital signals, i.e., the bit rate, to, e.g., 40 Gb/s.
From the literature, systems are known in which signals are transmissible at a bit rate of 10 Gb/s. From B. Wedding et al, "10 Gb/s to 260 000 Subscribers Using Optical Amplifier Distribution Network", Contribution for ICC/Supercom '92, Optical Communications 300 Level Session "Impact of Optical Amplifiers on Network Architectures", it is known, for example, to modulate a laser (high-speed multiquantum-well DFB laser) for this bit rate directly. "Direct modulation" means that the injection current of the laser is varied in accordance with the signal to be transmitted. In this manner, intensity or frequency modulation is possible. For this application, 10-Gb/s transmission, the DFB laser must be optimized to reduce undesired modulation characteristics. These undesired characteristics include the chirp effect (frequency instability which occurs during modulation) and parasitic impedances of the laser.
For these reasons, there is an upper limit of the bit rate above which conventional directly modulated lasers can no longer be used. An upper limit also exists for the drive electronics of the laser.